I noticed that on my installation (Docker version 19.03.3, build a872fc2f86) on Ubuntu 19.10, the db container kept failing. Examining the logs showed:
[ERROR] Could not open mysql.plugin table. Some plugins may be not loaded
After some digging, I found removing the --rpm option on mysql_install_db fixed this issue. Not sure what --rpm is needed for, so will look out for further issues.